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Simulation of a Polar Low South of Jan Mayen

(Bodil Aarhus and Elmer Raustein, GIUB)

On 27 Febr. 1984 a research aircraft flew through and gathered observations of a polar low that formed 200km south of Jan Mayen. The analyses of this low were made from these measurements, in addition to satellite and conventional synoptic observations. The observations from the aircraft were taken during the Arctic Cyclone Expedition (ACE) in 1984 (Shapiro et al., 1987). Those measurements were, as far as we know, the first detailed set of observations ever done of a polar low. In the following years, several attempts have been made to numerically simulate the development of this polar low (e.g. Gronas, 1986, dell'Osso, 1992). Qualitatively, these simulations were relatively successful, but the results lacked some essential features.

We have performed new simulations utilising objectively reanalysed data from the ECMWF as initial and boundary values. We used a modified version of the former Norwegian operational model NORLAM. The main modification being the replacement of the condensation scheme was replaced with that of Sundqvist (Sundqvist et al., 1989). The horizontal grid distance was 12.5 and 25km, respectively, with 30 levels in the vertical. In the following a brief description of the results will be given. Several (at least 3) mesoscale lows developed by so-called downstream development, the first of them being identified with the abovementioned polar low (hereafter called the ACE low). All the lows meet the criteria for being classified as polar lows. These polar lows developed downstream of a cold trough which was first connected to a synoptic scale/mesoscale low (the "mother low") between Iceland and Greenland, but then started to move faster than the "mother low". The trough moved eastward over a low-level baroclinic zone, when in turn it triggered the development of the polar lows. The trough did not phase-lock for a prolonged time to any of the polar lows. Instead, it moved faster than them and triggered a new deve- lopment further downstream.

In the figures, the surface low centered at appr. 69N, 8W is the ACE low. It has recently been under the influence of the trough, which has now triggered a new development centered at appr. 70.5N, 1W. The "mother low" has now disappeared, the northern front on the figure was connected to that low. It is also interesting to note that the frontel structure throughout the simulation corresponds very well with the Shapiro-Keyser conceptual model (Shapiro and Keyser, 1990). The position of the simulated ACE low agreed with the observed one to within 200km at 12Z, 27 Febr. 1984, while the minimum surface pressures were 986.5hPa and 979hPa, respectively. The diameters of the two lows were about 400km. A paper on these simulations is under preparation.

References:

dell'Osso, L. (1992). Extreme Oceanic Cyclogenesis. From an ECMWF Seminar in 1992.

Gronas, S., A. Foss and M. Lystad (1986). Numerical Simulations of Polar Lows in the Norwegian Sea. Part I: The Model and Simulations of the Polar Low 26-27 February 1984. Polar Lows Project. Techn. Report No. 5 DNMI, Oslo.

Shapiro, M., L. Fedor and T. Hampel (1987). Research Aircraft Measurements of a Polar Low over the Norwegian Sea. Tellus 39A, pp. 272-306.

Shapiro, M. and D. Keyser (1990). Fronts, Jet Streams and the Tropopause. Extratropical Cyclones, The Eric Palmen Memorial Volume, Am. Met. Soc. pp. 167-191.

Sundqvist, H., E. Berge and J. E. Kristjansson (1989). Condensation and Cloud Parameterization Studies with a Mesoscale Numerical Weather Prediction Model. Mon. Wea. Rev. 117. pp. 1641-1657.